In my current game as Assyria, I’ve formed a Tribal Alliance with the Numidians early on to try out assimilation of the tribe peacefully by settling near their settlements. It’s worked out well so far, as I have the entire south of my large island now protected by masses of mounted troops and I’ve already absorbed one of their settlements. I’m a bit concerned about keeping them peaceful because there are a LOT of horseriders down there.
Later, the Vandals, who went to war with me for no particular reason (they weren’t even nearby), created a Peace offering event where one option was to start an ambition of having a Tribal Alliance with any tribe, including peace with the Vandals. Strangely, the game doesn’t count my current Tribal Alliance as a trigger to complete that ambition (ambition is 0/1). It makes sense, except that you can’t have more than one Tribal Alliance at a time. I’m thinking that the game isn’t supposed to offer a tribal alliance as an ambition if you already have one because that’d force you to break an alliance just to start one again. Feels like it might be something that needs changing, but perhaps it’s designed that way purposefully.